1. Field of the Invention
The present invention relates to an imaging element, an imaging device and an endoscope system.
2. Description of the Related Art
Conventionally, a technique of providing a noise elimination unit for each pixel column has been known for imaging devices having complementary metal-oxide semiconductor (CMOS) image sensors, in order to eliminate a fixed pattern noise due to variation of transistors among pixels and a reset noise of charge-voltage conversion units within unit pixels (for example, see Japanese Laid-open Patent Publication No. 2000-059691 and Japanese Laid-open Patent Publication No. 2006-121652).
FIG. 12 is a circuit diagram illustrating a configuration of a conventional imaging device. In this example, a description will be made regarding a case in which an imaging device 500 has a CMOS image sensor.
The imaging device 500, for example, is arranged at a distal end portion of an endoscope and includes a light receiving unit and a reading unit. The light receiving unit is configured of a plurality of unit pixels 530, which are arranged in a two dimensional matrix form over a plurality of rows and a plurality of columns, and a plurality of vertical transfer lines 539, each of which is provided for each column of the two-dimensional matrix and transfers a signal output from each of the unit pixels 530. The reading unit is configured of a vertical scanning unit (row selection circuit) 541, a pixel drive line 549, which supplies a drive signal from the vertical scanning unit 541 to each of the unit pixels 530, a noise elimination unit 543, which is provided for each pixel column, and a horizontal scanning unit (column selection circuit) 558.
Each of the unit pixels 530 includes: a photoelectric conversion element, which accumulates a signal charge depending on the amount of incident light; a charge conversion unit, which performs voltage conversion on the signal charge transferred from the photoelectric conversion element; a transfer transistor, which transfers the signal charge from the photoelectric conversion element to the charge conversion unit; a reset transistor, which resets the signal charge transferred to the charge conversion unit; a row selection transistor; and an output transistor, which outputs an imaging signal to the vertical transfer line 539.
The reading unit turns the row selection transistor of an arbitrary row into an ON state by the vertical scanning unit (row selection circuit) 541 and reads out the imaging signal to the vertical transfer line 539. The read out imaging signal is input to the noise elimination unit 543 and a noise component thereof is eliminated. Thereafter, the imaging signal is output as image information to outside by the horizontal scanning unit 558.
FIG. 13 is a circuit diagram illustrating a configuration of the noise elimination unit of the imaging device illustrated in FIG. 12. The noise elimination unit 543 includes: a transistor 544 for sampling and holding, with one end side thereof connected to the vertical transfer line 539; a coupling condenser (AC coupling capacitor) CC with one end side thereof connected to the other end side of the transistor 544; a charge accumulation condenser (sampling capacitor) CS, which is connected between the other end side of the AC coupling capacitor CC and ground; and a potential clamp transistor 545, which is connected to a connection node SN between the AC coupling capacitor CC and the sampling capacitor CS. The connection node SN is connected to the horizontal scanning unit 558 including a column selection transistor.
First, the noise elimination unit 543 turns the transistor 544 for sampling and holding into an ON state at the time of pixel resetting, transmits a noise signal transferred by the vertical transfer line 539 using the AC coupling capacitor CC, turns the potential clamp transistor 545 into an ON state for a predetermined time period, and samples a noise signal level in the sampling capacitor CS. Thereafter, at the time of reading out the imaging signal, the imaging signal including the noise signal (light-noise sum signal) is transmitted by the AC coupling capacitor CC again. Since a voltage change of the imaging signal after the pixel resetting is transmitted, as a result, it is possible to take out, from the light-noise sum signal, the imaging signal from which the noise signal has been subtracted.
The noise elimination unit 543 illustrated in FIG. 13 requires two condensers of the AC coupling capacitor CC and the sampling capacitor CS for each pixel column. When the number of pixels increases, a size of the condenser becomes a constraint to make miniaturization of the imaging device difficult. In addition, a gain is reduced due to capacity division between the AC coupling capacitor CC and the sampling capacitor CS when the signal level is sampled, and thus, the S/N ratio deteriorates. In order to suppress such a problem, it is necessary to increase a size of the AC coupling capacitor CC, and then, the miniaturization of the imaging device becomes further difficult.
In order for the miniaturization of the imaging device, it is possible to consider decreasing a capacity of the sampling capacitor CS. In the CMOS image sensor, there is a case in which a leakage current is generated in the column selection transistor or the like so that a noise is superimposed on the imaging signal read out to a horizontal transfer line. The column selection transistor selects a pixel matrix for column by column and reads out the imaging signal to the horizontal transfer line, and thus, there occurs a time difference between a column read out at the first time and a column read out at the last time. Since the leakage current is accumulated and superimposed on the imaging signal during such a time difference, so-called shading such as generation of unevenness in luminance in a horizontal direction is generated. In a case where the capacity of the sampling capacitor CS is sufficient, it is possible to absorb the influence of noise caused by the leakage current or the like, but the influence of noise caused by the leakage current or the like on the imaging signal becomes greater and an image quality becomes worse as the capacity becomes smaller.
There is a need for an imaging element, an imaging device and an endoscope system capable of achieving miniaturization without deterioration in image quality.